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Why Joins?

Data is split across tables. Joins combine related data from multiple tables into one result.

Sample Tables

-- users: id | name    | email
-- orders: id | user_id | product | amount

INNER JOIN (Most Common)

Returns only rows that match in both tables.

SELECT users.name, orders.product, orders.amount
FROM users
INNER JOIN orders ON users.id = orders.user_id;

LEFT JOIN

Returns all rows from left table + matching rows from right. NULL if no match.

SELECT users.name, orders.product
FROM users
LEFT JOIN orders ON users.id = orders.user_id;
-- Shows ALL users, even those with no orders

RIGHT JOIN & FULL JOIN

-- RIGHT JOIN: all rows from right table
SELECT * FROM users RIGHT JOIN orders ON users.id = orders.user_id;

-- FULL JOIN: all rows from both tables
SELECT * FROM users FULL JOIN orders ON users.id = orders.user_id;

Table Aliases

SELECT u.name, o.product, o.amount
FROM users u
JOIN orders o ON u.id = o.user_id
WHERE o.amount > 100;

✅ Quick Quiz

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